An interconnect circuit board is the physical realization of electronic circuits or subsystems from a number of extremely small circuit elements electrically and mechanically interconnected. It is frequently desirable to combine these diverse electronic components in an arrangement so that they can be physically isolated and mounted adjacent one another in a single compact package and electrically connected to each other and/or to common connections extending from the package.
Complex electronic circuits generally require that the circuit be constructed of several layers of conductors separated by insulating dielectric layers. The conductive layers are interconnected between levels by electrically conductive pathways through the dielectric called vias.
One well known method for constructing a multilayer circuit is by co-firing a multiplicity of ceramic tape dielectrics on which conductors have been printed with metallized vias extending through the dielectric layers to interconnect the various conductor layers. (See Steinberg, U.S. Pat. No. 4,654,095.) The tape layers are stacked in registry and pressed together at a preselected temperature and pressure to form a monolithic structure which is fired at an elevated temperature to drive off the organic binder, sinter the conductive metal and densify the dielectric. This process has the advantage over classical "thick film" methods since firing need only be performed once, saving fabricating time and labor and limiting the diffusion of mobile metals which can cause shorting between the conductors. However, this process has the disadvantage that the amount of shrinkage which occurs on firing may be difficult to control. This dimensional uncertainty is particularly undesirable in large, complex circuits and can result in misregistration during subsequent assembly operations.
Pressure sintering or hot pressing, the firing of a ceramic body with an externally applied load or weight, is a well known method for both reducing the porosity of and controlling the shape (dimensions) of ceramic parts. (See Takeda et al., U.S. Pat. No. 4,585,706; Kingery et al., Introduction to Ceramics, p. 502-503, Wiley, 1976.) Pressure sintering of ceramic circuits in simple molds is made difficult by the tendency for the part to adhere to the mold and/or for cross contamination to occur between the part and the mold. Further, application of a load or similar constraining force to the surface of a ceramic part during burnout of the organic binder may restrict the escape of volatiles, causing incomplete burnout and/or distortion.
Copending U.S. application, Ser. No. 07/466,934, discloses a method for constrained sintering that permits escape of volatiles during burnout of the organic binder. A release layer is applied to the surface of the unfired ceramic body. A weight is subsequently placed on the release layer to reduce shrinkage in the X-Y direction. The release layer between the weight and ceramic body provides pathways for the volatiles to escape. If a method were established whereby ceramic circuits could be constrained-sintered without need for a mold, without applying external loads, and without restricting the escape of volatiles during burnout, and yet still largely eliminate dimensional uncertainty in the final circuit, processing steps associated with firing the circuitry with reduced shrinkage could be simplified or eliminated. The advantage would be greater yet if the method would permit co-firing of conductive metallic pathways on the outer surfaces of the ceramic circuit.
Flaitz et al. (European Patent Application 0 243 858) describe three approaches to circumventing the aforementioned difficulties. With the first approach, constraint is applied only to the outer edges (periphery) of the part, providing an open escape path for volatiles and an entry path for oxygen. With the second approach, a co-extensive force is applied to the entire surface of the piece to be sintered by either using co-extensive porous platens or by application of an air-bearing force to the surface or surfaces of the piece to be sintered. With the third approach, a frictional force is applied to the sintering body by use of contact sheets comprised of a porous composition which does not sinter or shrink during the heating cycle and which prohibit any shrinkage of the substrate. The composition of the contact sheets is selected so that they remain porous during firing, do not fuse to the ceramic, are thermally stable so that they will not shrink or expand during the sintering cycle, and have continuous mechanical integrity/rigidity. The contact sheets maintain their dimensions during the sintering cycle, thus restricting the ceramic part from shrinking. After lamination of the contact sheets to the article to be sintered, sintering takes place without use of additional weights.